M. Oka

Non-mesonic weak decays of light hypernuclei in the direct quark and the one-pion exchange mechanisms

Abstract Contributions of the direct quark mechanism are studied in the non-mesonic weak decays of light hypernuclei. The ΛN → NN transition is described by the one pion exchange mechanism and the direct quark mechanism, induced by the four-quark vertices in the effective weak Hamiltonian. By employing a realistic wave function of the Λ inside the hypernuclei, non-mesonic decay rates of Λ4H, Λ4He, and Λ5He are calculated. The results show that the direct quark mechanism is significantly large and gives a large ΔI = 3 2 contribution in the J = 0 channel. The relative phase between the one-pion exchange and the direct-quark contributions is determined so that the effective weak Hamiltonian for quarks gives both of them consistently. We find that the sum of these two contributions reproduce the current available experimental data fairly well.

Pentaquark baryon in anisotropic lattice QCD

The penta-quark(5Q) baryon is studied in anisotropic quenched lattice QCD with renormalized anisotropy a_s/a_t=4 for a high-precision mass measurement. The standard Wilson action at beta=5.75 and the O(a) improved Wilson quark action with kappa=0.1210(0.0010)0.1240 are employed on a 12^3 times 96 lattice. Contribution of excited states is suppressed by using a smeared source. We investigate both the positive- and negative-parity 5Q baryons with I=0 and spin J=1/2 using a non-NK-type interpolating field. After chiral extrapolation, the lowest positive-parity state is found to have a mass, m_{Theta}=2.25 GeV, which is much heavier than the experimentally observed Theta^+(1540). The lowest negative-parity 5Q appears at m_{Theta}=1.75 GeV, which is near the s-wave NK threshold. To distinguish spatially-localized 5Q resonances from NK scattering states, we propose a new general method imposing a ``Hybrid Boundary Condition (HBC), where the NK threshold is artificially raised without affecting compact five-quark states. The study using the HBC method shows that the negative-parity state observed on the lattice is not a compact 5Q but an s-wave NK-scattering state.

Effective potential of the O(N) linear sigma-model at finite temperature

Abstract:We study the O(N) symmetric linear sigma-model at finite temperature as the low-energy effective models of quantum chromodynamics (QCD) using the Cornwall-Jackiw-Tomboulis (CJT) effective action for composite operators. It has so far been claimed that the Nambu-Goldstone theorem is not satisfied at finite temperature in this framework unless the large-N limit in the O(N) symmetry is taken. We show that this is not the case. The pion is always massless below the critical temperature, if one determines the propagator within the form such that the symmetry of the system is conserved, and defines the pion mass as the curvature of the effective potential. We use a regularization for the CJT effective potential in the Hartree approximation, which is analogous to the renormalization of auxiliary fields. A numerical study of the Schwinger-Dyson equation and the gap equation is carried out including the thermal and quantum loops. We point out a problem in the derivation of the sigma meson mass without quantum correction at finite temperature. A problem about the order of the phase transition in this approach is also discussed.

Chiral unitary approach to the πN∗N∗ , ηN∗N∗ couplings for the N∗(1535) resonance

Abstract Using a chiral unitary model in which the negative parity nucleon resonance N ∗ ≡N ∗ (1535) is generated dynamically by means of the Bethe–Salpeter equation with coupled meson–baryon channels in the S=0 sector, we have obtained the π 0 N ∗ N ∗ and ηN ∗ N ∗ couplings. The π 0 N ∗ N ∗ coupling has smaller strength but the same sign as the π 0 NN coupling. This rules out the mirror assignment of chiral symmetry where the ground state nucleon N and the negative parity resonance N ∗ are envisaged as chiral partners in the baryon sector.

Mixings of four-quark components in light nonsinglet scalar mesons in QCD sum rules

Mixings of four-quark components in the nonsinglet scalar mesons are studied in the QCD sum rules. We propose a formulation to evaluate the cross correlators of

Decays of12−baryons in chiral effective theory

qoverline{q}

Properties of N∗(1535) at finite density in the extended parity-doublet models

and

Bosonic excitations in a crystal of sine-Gordon kinks

qqoverline{q}overline{q}

eta --> pi 0 gamma gamma decay in the three-flavor Nambu-Jona-Lasinio model.

operators and to define the mixings of different Fock states in the sum rule. It is applied to the nonsinglet scalar mesons,

Dynamical chiral symmetry breaking in effective models of QCD in the Bethe-Salpeter approach

{a}_{0}